1. Field of the Invention
The invention relates to a valve for controlling fluids and particularly to an improved valve having an actuator piston actuated by a piezoelectric actuator.
2. Description of the Prior Art
One such valve is known from European Patent Disclosure EP 0 477 400 A1, for instance. There, the actuating piston of the valve member is disposed displaceably in a smaller-diameter part of a stepped bore, while conversely a larger-diameter piston, which is moved by a piezoelectric actuator, is disposed in the larger-diameter part of the stepped bore. Enclosed between the two pistons is a hydraulic chamber, such that when the larger piston is moved by the piezoelectric actuator, the actuating piston of the valve member is moved by a distance that is increased by the boosting ratio of the stepped bore diameters. The valve member, the actuating piston, the larger-diameter piston, and the piezoelectric actuator are located successively on the same axis.
If a piezoelectric actuator is to be usable as a control element, the change in length as a function of temperature must be compensated for. Since the stroke attainable by means of a piezoelectric actuator amounts to only between about 1/1000 and 1.5/1000 of its length, for many applications it is necessary for this tiny stroke to be boosted. The different coefficients of thermal expansion of the various materials used cause settling effects, which are sometimes greater in the stroke direction than the possible stroke caused by the piezoelectric actuator element.
To create a compensation for such variations, in the valve disclosed in EP 0 477 400 A1, a defined leak is provided in the hydraulic chamber. Upon slow changes in the valve structure, of the kind that can be caused temperature changes, for instance, the hydraulic fluid can escape through the leak and thus compensate for the effects in the stroke direction. The viscosity of the hydraulic fluid is selected such that upon rapid changes, of the kind caused by the piezoelectric actuator, the hydraulic fluid does not escape through the leak, and the deflection of the piezoelectric actuator is transmitted to the actuating piston. This compensation is very complicated and expensive, because it requires very close tolerances in the production of the pistons in order to be able to create a defined leak in the form of an annular gap between the piston and the surrounding cylinder wall. Furthermore, diverted hydraulic fluid must be returned to the hydraulic chamber again, for which purpose suitable devices must also be provided.
The valve for controlling fluids according to the invention has the advantage over the prior art that it is very simple in structure and can be produced economically. A ratio of coefficients of thermal expansion of approximately or equal to 1 is understood to mean values between 1.0 and approximately 1.1. In the ideal case, the ratio is 1.
In the valve of the invention, this means a markedly reduced number of parts. As a result, putting together the valve is simplified, and calibration operations are fewer, since fewer parts have to be calibrated. This means in particular that the production and assembly costs for the valve can be reduced markedly.
In a preferred exemplary embodiment of the invention, the compensation element is embodied as a cylindrical ring element, which surrounds the piezoelectric actuator. By means of this design of the compensation element, an optimal compensatory effect can be assured, since the piezoelectric actuator and the compensation element are exposed to the same temperature effects. This design of the compensation element also dictates a simple design of the valve of the invention.
Alternatively, it is also possible in an advantageous embodiment to dispose the compensation element merely parallel to the piezoelectric actuator. As a result, on the one hand the compensation element can assume various shapes, for instance cylindrical, or triangular or square in cross section, and so forth. In this respect, in terms of its shape the compensation element can be adapted to the spatial conditions of the valve design. By this means it can also be assured that the piezoelectric actuator and the compensation element are always located quite close together, so that the temperature factors affect both elements to the same extent.
In a further feature, the piezoelectric actuator and compensation element are disposed spatially near one another, preferably in a common chamber. In that case, temperature changes act in the same way on both parts, so that the changes in length of the piezoelectric actuator and the compensation element compensate for one another.
If the coefficients of thermal expansion of the piezoelectric actuator and compensation element are the same, then advantageously a design is selected in which the effective length of the compensation element is equivalent to the length of the piezoelectric actuator. The term xe2x80x9ceffective lengthxe2x80x9d is understood to mean the expansion of the compensation element parallel to the axis of the piezoelectric actuator that is available for an expansion of the compensation element in the direction of the axis of the piezoelectric element.
It has proved to be suitable for the operation of the valve if the compensation element comprises Invar(copyright).
In an advantageous embodiment, an air gap is provided between the transmission element and the booster. The air gap amounts to only a few micrometers. If the piezoelectric actuator and the compensation element do not have precisely the same coefficient of thermal expansion, then in this way a compensation for residual error can be achieved.
Advantageously, the transmission element includes a tie rod, and the compensation element is part of the tie rod. In this way, the transmission element is very simple to produce, and only slight problems from production variations occur.
A sturdy embodiment of the valve is attained if the booster is embodied as a mechanical booster, preferably as a lever.
In an advantageous feature of the invention, a support point of the lever is located in the axis of the piezoelectric actuator.
If the coefficients of expansion of the piezoelectric actuator and compensation element are not precisely the same, or if a longitudinal expansion of other materials is to be compensated for along with the longitudinal expansion of the piezoelectric actuator, then it is advantageous if the effective length of the compensation element is not equal to the length of the piezoelectric actuator.